1. Field of the Invention
The present invention relates to microwave circuits and, more specifically, to microwave filters.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art and/or what is not in the prior art.
A band-stop or band-rejection filter is a filter that passes substantially unaltered most frequencies in its spectral range of operation, except for the frequencies in one or more specific spectral bands (stop bands), which are attenuated to a relatively low level. A band-stop filter performs a spectral function that is substantially opposite to that of a corresponding band-pass filter. A band-stop filter having a relatively narrow stop band is often referred to as a notch filter. Tunable band-stop filters find applications in communication systems, for example, at a receiver, to remove interference signals originating from co-located transmitters and/or from adjacent receive bands and, at a transmitter, to remove harmonic and spurious signals, e.g., caused by power-amplifier nonlinearities.
A microwave photonic filter is an optoelectronic (or electro-optic) circuit designed to perform functions that are analogous to those of a conventional microwave filter. As used in this specification, the term “microwave” designates electromagnetic signals having frequencies in the range from about 3 Hz to about 300 GHz. As such, this term covers radio-frequency (RF) signals and millimeter-wave signals in addition to what is traditionally referred to as microwave signals.
Microwave photonic filters have certain recognized advantages over conventional microwave filters. These advantages include, but are not limited to, a relatively low loss that is substantially independent of the signal frequency, relatively low sensitivity to electromagnetic interference (EMI), relatively low weight and small size, and amenability to spatial and spectral parallelism through the use of wavelength-division multiplexing (WDM) techniques. Implementing microwave photonic filters with standard silicon complementary-metal-oxide-semiconductor (CMOS) technology holds the promise of minimizing production costs, e.g., through monolithic integration of electronic and photonic functions and the use of the massive existing CMOS manufacturing infrastructure.